Interval 244-360 mbsf

Below the S 1tS3 (Fig. 2.1) unconformity, core recovery improved from 2.3% to 34% because of a change in matrix induration. We measured laboratory compressional wave velocity data on board ship using the PWS3 contact probe system for specimens and split cores (Fig. 2.3;

Shipboard Scientific Party, 1999b: fig. F24, PWS3 data for Site 1103).

Preliminasy comparisons of velocity data and lithological descriptions (Eyles et al., 2001) suggest that all recovered lithological changes are represented within the velocity data measured. We still have no information for 60% of the core. In order to produce a continuous velocity log for subsequent Users, we made, the following assumptions based On the available data: The lab velocity (Fig. 2.3) and density data (Shipboard Scientific Party, 1999b), indicate that low-recovery zones are commonly located at acoustic impedance changes. We therefore assume that all major impedance changes are represented within the available data.

Furthei-more, in order to reduce data gaps by two thirds of their depth interval, we added artificial data points at both ends of the gap (Fig. 2.3).

This measure is supported by the observation of discrete changes in sedimentology within the cores recovered (interchange of clast-sich structureless diamictite with more sorted sands and silts). Using a simple intespolation technique to fill the data gaps would have caused unrealistic continuous transitions within the velocity profile that are also unfavourable for later seimic modelling (e.g. the calculation of synthetic seismograms).

CHAPTER 2: The West Antarctic Shelf

2.2.4 Results and Discussion

Data "clean up" for Site 11 03 log velocities

Fig. 2.1. Example of data reduction using polynomial fitting with cut-off limits of various orders. For a detailed description of the techniques refer to Shipboard Scientific Party, 1999a.

Elliptic IIR-filter, 0-phase, order 12

Frequency (11m)

Fig. 2.2. A specially designed low-pass filter is used to remove short-wavelength variations. As a consequence, the depth resolution is reduced to

-

^{2 m.}

CHAPTER 2: The West Antarctic Shelf

Tab. 2.1. To synchronize the new velocity data to other logging data of the Borehole Research Group at Lamont- Doherty Earth Observatory (BRG-LDEO), we used 14 prominent features of the IMPH resistivity log. The synchronization was achieved with the linear mode of AnalySeries 1.2 (Paillard et al., 1996).

All the data derived from the previously described 13 categories of processing techniques (without the laboratory-derived velocities) incorporated into the initial data pool from 0 to 244 mbsf are shown in Fig. 2.3. The data are also given in Tab. 2.3. We merged all data categories, with a total of 5400 values into a single depthlvelocity matrix by offsetting the depth of the individual velocity categories by 5 mm. Looking at the hole section in a compressed representation, it is difficult to detect well-supported trends (Fig. 2.3). Although only data that passed several quality criteria were included, the data Set remains extremely spiky and velocity variations of 1600 to 2800 rn/s for the same depth interval are conmon.

Based on the large standard deviation of the data, we rejected a simple smoothing of the values. Our first step toward simplifying the data was carried out in 25-m sections. We used our own method as described in ODP Leg 178, Initial Reports, Seismic Stratigraphy,

CHAPTER 2: The West Antarctir Shelf

'Explanatory Notes" (Shipboard Scientific Party, 1999a). This method is based on individual polynomial fittings of variable orders and uses selectable confidence intervals around the polynomial fit. An example of a six-step clean up is given in Fig. 2.1. An interpolated trendline connecting values chosen is shown together with the initial data pool in Fig. 2.3 (blue line). It has to be noted that the polynomial fitting and exclusion technique will always prefer incorporating regions with high data density. In most cases this is an improvement over simple averaging since outliers are completely removed and do not affect the resulting data.

On the other hand, this method by no means ensures the extraction of only good data out of clusters in case the majority of the data for one depth interval is ersoneous and the described data separation technique fails.

Subsequently, the data were filtered with a low-pass filter (Fig. 2.2) designed to exclude short wavelength variations. The frequency range of the pass band is set to reduce the vertical resolution of the filtered velocity log to approximately 2 m. The final representation of our approach is given in Fig. 2.3. Since all data processing within this study is based on raw unsynchronized data with respect to depth shifts between logging runs and with respect to the different transmitter receiver pairs used for the different data categories a final depth match was necessary. To achieve a profile comparable to other depth-shifted logging data processed by the Borehole Research Group at Lamont-Doherty Earth Observatory (BRG- LDEO), the resulting plots were graphically fitted with the IMPH resistivity data. The program used is AnalySeries 1.2 (Paillard et al., 1996), and the 14 matchpoints and resulting depth) (shifts are given in Tab. 2.1 and Fig. 2.3. The resulting depth shifts are larger near the drill pipe, between 87 and 100 mbsf according to the BRG-LDEO depth scale, very reasonable in the intervals 100 to 207 mbsf and 230-244 mbsf and unrealisticly high in the short interval between 207 and 212 mbsf. The higher shift values at the base of the drill pipe may be due to problems encountered during the process of reentering the tools after the logging run (Shipboard Scientific Party, 1999b).

Variable shift values between 0 and 3 m can be easily explained with a combination of three effects:

1) During the data processing at BRG-LDEO the GHMT log was used as the reference for depth-matching. The maximum depth shift of the Triple Combo (including the resistivity log) relative to the reference log was between 1 and 2 m. The maximum shift applied to the two FMS-sonic logging runs was an additional 0.6 m.

CHAPTER 2: The West Aiztarctic Shelf

2) The maximum receiver transmitter spacing on the sonic tool is 13.5 ft (-4.1 m).

During data processing the traveltime information of different transmitter receiver pairs with different spacings and effective integrative depth range was brought together without calibrating the sensor-pairs to their effective depth and without synchronizing the two logging runs individually beforehand.

3) The low pass filtering of the data is accompanied by a reduction of the depth resolution

Site 1103A, Leg 178 T i e points"

Resistivity Velocity

(nm) (m/si

Fig. 2.3. Composite velocity profile of Site 1103, Leg 178. The different data resources are indicated. A digital version of the composite data can be found in Tab. 2.4.

CHAPTER 2: The West Antarctic Shelf

Velocity composite profile

Leg 178, Site 1103

Velocity (M's)

-

^Seafloor

- low recovery - degraded logs

-

End of pipe

during logging

- low recoveiy - logging dafa avalible

Start of core recovery

- improved recovery - la borafory velocif~es

a vaiia bie - no logs

1

^{@ S ~ t e} 1403 laboratory velocit!es Average velocity of Site 739, Leg 119

1

Stte 1100 .'Top Sets" laboratory v e l m t t ~ ? ~ Filtered and interpolated velocity

Fig. 2.4. The reduced and filtered logging velocity data synchronized with the depth-shifted resistivity data integrated resistivity [IMPH] and self focusing resistivity [SFLU] processed by the Borehole Research Group at Lamont-Doherty Earth Observatory (BRG-LDEO). Tie points are marked with crosses. Absolute shifts are given in Tab. 2.1.

CHAPTER 2: The West Antarctic Shelf

Fig. 2.5. Comparisons of the (A) new velocity data and representative dowiihole logs: (B) APLC (neutron porosity), (C) SFLU (electrical resistivity), (D) RHOM (bulk density), and (E) RMGS (magnetic susceptibility).

The logging units I-V as defined by the Shipboard Scientific Party (1999b) are outlined (E). Note the difference in depth for Fig. 2.6A and Fig. 2.6B-E.

CHAPTER 2: The West Antorctic Shelf

NW Seismic profile 195-1

S E

Fig. 2.6. Comparisons of the (A) time and (B) depth sections of line 195-152 in the vicinity of Site 1103 (Shipboard Scientific Party, 1999b). The depth migration is based on the new velocity data. Note the differences in depth scale and apparent SlIS3 geometry between (A) the time section of the Initial Reports volume with an approximate depth annotation, and (B) the depth migrated section presented by this study. The presented data confirms the location of the shelf unconformity SllS3 at 222 ms TWTT or 243 mbsf. The unconformity is seismically expressed by a strong negative and subsequent positive reflection. Refer to Shipboard Scientific Party (1999b) for a detailed description of seismic units and major reflectors (a-e) shown in (A).

In the final velocity profile the effect of these three factors is combined, depending on the importance of each data category for a specific depth interval to the finally chosen data.

However, the large shifts in the depth interval 207 to 212 mbsf are probably unrealistic and the result of a missmatch or incorporation of erratic data into the final velocity data selection.

The correlation is based entirely On graphical correlation with the Same systematics for the whole section (matching regional highs and lows of the reference with regional highs and lows of the filtered data curve). Given that the erroneous interval is very short, we decided accept the con'elation and accepted its limitations in the mentioned depth interval 207-212 mbsf.

Finally, after compiling the data from all depth sections (0-75, 75-244, and 244-360 mbsf), the contacts were smoothed, resulting in a continuous profile displayed together with the laboratory velocity data values in Fig. 2.4.

The computed velocity curve can be compared for validation to the other representative downhole logs obtained at Site 1103 (Shipboard Scientific Party, 1999b): the neutron porosity (APLC), the bulk density (RHOM), the electrical self-focusing resistivity (SFLU), and the

CHAPTER 2: The West Antarctic Sl7ey

magnetic susceptibility (RMGS) (Fig. 2.5). The chosen logs show reliable values, except the anomalies in the RMGS log at -1 17 mbsf, caused by the APS bow spring lost in the hole.

The velocity curve is generally con'elated with the RHOM, RMGS, and SFLU logs, and anti-correlated with the APLC log. The velocity curve shows the Same features as the other logs that are divided in five units (Fig. 2.5). The first unit is characterized by low porosity and high resistivity, density, and velocity values. The second unit exhibits porosity values between 25% and 50% and lower susceptibility, density, velocity, and resistivity values. The two thin beds (-132 and -142 mbsf) seen in all the logs (high SFLU and RHOM values; low NPHI and RMGS values) are also found in the velocity curve. In the third unit, the resistivity, susceptibility, and velocity logs all show higher values at the top with a slight tendency to decrease down the hole. The fourth unit is characterized by a sharp reduction of the logged values and a sharp increase in the porosity. In the last unit, we note a distinct jump to lower density, resistivity, susceptibility, and velocity values and higher porosity. The high variability of the logs in this past of the section is not Seen in the velocity curve, probably because of the smoothing method used to reconstruct it.

Fig. 2.6 shows a compasison of the original seismic data and a depth-migrated section generated by using the new velocity data. The original time section already published in the Initial Reports Volume, ODP Leg 178 (Shipboard Scientific Party, 1999). The log data confirm the location of the major shelf unconformity at 222 ms two-way traveltime (TWTT) or 243 mbsf (see Escutia et. al., in prep., for a thorough discussion of the nature and history of the unconforrnity). The unconformity between seismostratigraphic units S l and S3 (Shipboard Scientific Party, 1999b) is seismically expressed by a strong negative and subsequent positive reflection arround 222 ms TWTT below sea floor (Fig. 2.6A). The decline and rise in acoustic impedance (acoustic impedance = velocity X density) within the depth interval 220 to 245 mbsf Seen in the velocity and density data of Fig. 2.5D are most likely the cause of this reflector. The positive reflection around 206 ms TWTT, is probably still part of the S l topset package.

2.2.5 Conclusions

Starting with nonconclusive velocity logs caused by a slow formation with extremely high internal velocity contrasts, and an uncentered logging toolstring without bow springs, we improved the quality of the data and evaluated data previously unavalible with standard processing techniques. The velocity profile we produced correlates well with the other logs obtained (neutron porosity, bulk density, self-focusing electrical resistivity data, and magnetic

CHARTER 2: The West Antarctic Shelf

susceptibility; Fig. 2.5) and offers a reasonable estimate for the location o f a prominent shelf unconformity. However, we must emphasize the limitations of the data. Esrors may be introduced by:

a bias in choosing and defining the data categories

the possibility o f exclusion o f rare good data in a given interval where misleading values represent the majority o f the data

uncertainity o f the precise depth o f data from transmitter and receiver spacings collecting different regions along the toolstring for a given tool position

mixing of the non depth-shifted raw data of logging runs one and two

bias in the final correlation by assuming that low resistivity zones are most cornmonly denser and therefore acoustically faster

Nevestheless in contrast to seismically derived velocity information (Tinivella et al., 2001, See chapter 2.3). The velocity information presented here is more detailed and allows the investigation o f the seismic character at least on the scale o f the defined logging units (Fig. 2.5). W e hope that the new velocity model will help seismostratigraphers, modelers, and sedimentologists to understand complexities o f the Antarctic shelf. The new infor-mation is utilized in the paper by Escutia, et al. (in prep.).

2.3 Validation and Application of the new Velocity Data

2.3.1 Comparisons o f the new velocity data with other velocity information

Prior to Leg 178, as past o f the site survey and requested by the ODP Site Survey Panel, the Prograrnrna Nazionale di Ricerche in Antastica provided P-wave interval velocities for all drill sites. These stacking velocities have been supplemented post cruise by tomographically derived velocities and submitted to the ODP Leg 178 Scientific Results Volume (Tinivella et al., 2001).

Thanks to the authors (Tinivella et al., 2001) who provided the data for profile 195-152 prior to publication, it is possible to compare the velocities based on logging and lab measurements developed in this thesis with the velocities derived from multichannel seismic survey (MCS) data processing. Interval velocities derived from stacking velocity infosmation via Dick's fo~mula using root mean Square velocity infosmation constitute the prime seismic

CHAPTER 2: The West Antarctic Shelf

velocity information obtainable from MCS. Tinivella et al. (2001) describe the disadvantages of using stacking derived interval velocities for target reflector depth determination:

short streamer length compared to target depth reduces available offset and decreases data quality

the assumption of horizontal layered reflectors with no lateral velocity variation is commonly not attained in reality

Fig. 2.1. Comparison of different P-wave velocities available at Site 1103. Stacking and tomographic velocities are adapted from Tinivella et al. (2001).

To obtain additional independent velocity information Tinivella et al. (2001) use iterative modelling based on ray tracing (seismic tomography). Semi-manually picked reflectors define a starting velocity model; this is then varied in geometry and velocity until lateral dispersion of resulting reflectors is brought to a minimum. This type of modelling also considers lateral velocity variations along reflector packages. The resolution of a velocity profile derived by tomography may be of the order of the trace spacing (12.5 m for profile 195-152).

The velocities for Site 1103 ase illustrated in Fig. 2.1. In case of the tomography velocity (TV), the topset Unit S l (Fig. 2.1 and Fig. 2.1) is divided into 3 layers of increasing velocity with depth. No sharp increase in TV is found at the lower boundary of the topset Unit

5 8

CHAPTER 2: Tlze West Antarctic Shelf are dramatically lower in Unit S 1. The SV velocities seem to represent a sunning average for Unit S l and S3. Neither TV nor SV is able to represent velocity inversions in the 20-30 m range, notably the velocity inversion below the shelf topsets Unit S 1.

Tinivella et al. (2001) describe one case (a location rnidway between Site 1100 and Site 1103, Fig. 2.1), where an inversion in the tomographic velocities occurs at the S2/S3 boundary. They attribute this inversion to ice load induced overcompaction of S l and S2 compared to a normally compacted S3. The fact. that no inversion occurs in the tomographic velocities at Site 1103 is attributed to the nature of the contact (near-conformable Site 1103 vs. erosional at the 1100/1103 midway position).

One can disagree with this interpretation since the new velocity profile for Site 1103 shows a clear inversion at the top of S3. Seismic tomography is not detailed enough to show small-scale variations in the 20-30 m range. Also, the link between overcompaction and seismic velocity is weak, since lithological changes within Unit S3 (see chapter 2.4) are able to produce velocity variations exceeding those suspected from compaction history differences. Unit S3 is lithologically different from Unit S l , which consists of loose gravel in a claylsand matrix, and the sediments of Unit S3 are diagenetically cemented. For acoustic behaviour the presence of cement in the porespace is more relevant than the compaction history. The velocity inversion (also seen as a density inversion in the logs, Fig. 2.5 and Fig.

2.1) at the top of Unit S3 seems to be a random feature of the stratal location of the unconformity separating S l and S2 from the underlying Unit S3. Along the seismic profile, there are locations where S 1 or S2 are in unconformable contact with a high velocity layer of S3 (cemented diamicts), which result in a positive velocity step. Another possible explanation for a velocity inversion could be the presence of a reworked high porosity horizon at the base of Unit S 1 or S2, consisting of unrecovered, reworked, cemented older strata, (Fig. 2.1). In the later case the inversion Zone should be restricted to the overlaying strata.

Overall, the SV seem to be much more reliable than TV in indicating absolute vertical velocity distributions. This is especially true where the streamer had a length of 1500 m and the target reflector depth was between 500 and 800 m.

CHAPTER 2: The West Antarctic Shelj

2.3.2 Site 1103 Synthetic Seismogram 2.3.2.1 Background

As a quality check of the new velocity profile derived with unusual processing techniques of logs and lab data, Trevor Williams of the Borehole Research Group at Lamont-Doherty Earth Observatory calculated a synthetic seismogram on the base of this new velocity information.

The figures shown will be published in a joint paper with Escutia and Lauer-Larede outside ODP.

A B C D E F G H

Fig. 2.1. Input variables and processing steps of a synthetic seismogram for Site 1103. Note that TWTT is given starting from the seasurface compared to Fig. 2.1 and Fig. 2.6 where the timescale starts at the sea floor offset

-

700 ms). See text for details. Figure received and modified from Trevor Williams of the Borehole Research Group at Lamont-Doherty Earth Observatory (LDEO).

CHAPTER 2: The West Antarctic Shelf

Site 1103 Shot points

1665 1675 1685 1695 1705 1715 1725 1735 1745

Fig. 2.2. The new synthetic traces overlain on part of MCS line 195-152. The major reflectors are in place -

indicating an overall correct velocity model. Note that TWTT is given in sbss (seconds below sea surface), see notes Fig. 2.1. See text for detailed discussion of the comparison. Figure received and modified from Trevor Williams of the Borehole Research Group at Lamont-Doherty Earth Observatory.

The synthetic traces are calculated with a professional Schlumberger package (ESX of GEOQUEST) and not with the software synseis described in the "Excursus" (chapter 7.1.7).

Starting parameters for the caiculation (see the "Excursus", chapter 7.1.6 for mathematical and physical details) are the new velocity profile, a density profile (both shown in the second column from left (Fig. 2.1B) and a source wavelet (Fig. 2.1F).

Density values are empirically derived from deep penetrating resistivity data (SFLU, Fig. 2.3 and Fig. 2.5) and index properties of discrete samples (below 243 mbsf, when recovery started). The first step is an empiric conversion to porosity (Eq. 2.3, oral communication T. Williams, 2001) followd by a transformation to density using fixed density values for porewater (1.03 g/cm3) and matrix (2.75 g/cm3).

CHAPTER 2: The West Antarctic Shelf

0.28 Porosity =

SFLU Resistivity (Eq. 2.1)

The wavelet is extracted via the ESX package from 20 neighbouring traces in the vicinity of Site 1103 using built-in deconvolution algorithms. Note the extreme length of the signal (0.06 sec or 90 meters in seawater). The resulting reflectivity coefficient profile (Fig.

2.1E), the synthetic traces after convolution (Fig. 2.IG) and adjacent traces from profile 195- 152 are also given (Fig. 2.1 H). data are entirely log based, because recovery was poor in this interval.

Beside gaps in the data, there are some features in the seismic section that ase not

Beside gaps in the data, there are some features in the seismic section that ase not

Im Dokument 1-3 (Seite 62-0)